Release Date: June 1981
Released By: AT&T's Unix Support Group (USG)
Source Code: No known source code preserved
Documentation: UNIX User's Manual, UNIX Administrator's Manual, Documents for UNIX

UNIX Release 4 was a major release of the PWB/UNIX line in the Bell System in 1981. Unlike the prior and following versions which were released as System III and System V respectively, there was no external release of this version as UNIX System IV.

Many of the changes which were first encountered outside of the Bell System in UNIX System V truly had their start in Release 4. Examples of such features which originated in Release 4:

• The Common Object File Format (COFF)
• The LP Print System
• Semaphores, Shared Memory, and Message Handling (IPC) matching System V introduced in 4.1
• The System Generation Software (SGS)
• The termio terminal handling framework

Current documentation proven to exist for Release 4 include:

• Documents for UNIX Release 4.0 (https://www.tuhs.org/Archive/Documentation/Manuals/Unix_4.0/)
• UNIX Reference Guide Release 4.0 (https://www.tuhs.org/Archive/Documentation/Manuals/Unix_4.0/UNIX_Reference_Guide.pdf)
• UNIX User's Manual Release 4.1 (3B20) (https://gitlab.com/segaloco/pwb4u_man)
• UNIX Programming Starter Package (https://archive.org/details/unix-programming-starter-package)
• UNIX Text Editing and Phototypesetting Starter Package (Archival in Progress)
• Note: the two above “Starter Package” documents are simply a subset of the Documents for UNIX set, so the content as of present is available in that collection. There are later versions of these manuals which swap out Bell Labs icons and labeling with AT&T. It is unknown at present whether these are simply cover facelifts or represent revisions to the papers as well.

Additional documentation cited in the above:

• UNIX Administrator's Manual Release 4.1 (3B20)
• UNIX User's Guide

Note: This is material that is, as of this typing, being actively uncovered and investigated, so this content will be in flux as time goes on.

Nomenclature of this release is still being discussed. The official name in literature of this line for 3, 4, and 5 is simply “UNIX Release X” where X is the major and minor version number. Release 4 should be seen as distinct from 1973's research V4 as well as 1980's 4BSD.

The major version numbering scheme derives from PWB/UNIX as evidenced in this thread: https://www.tuhs.org/pipermail/tuhs/2019-September/018452.html